What Is an Agent Harness?

Point Obversa at an agent harness; you don’t write one to use it. An agent harness is the software around a model that turns it into an agent: the loop that reads the model’s answer and acts on it, the tools it may call, the memory it keeps between turns, and the guardrails. Claude Code, Codex, OpenCode and Grok’s CLI are agent harnesses. The layer that runs several of them as a team is the neighbour term, a meta-harness.

In Obversa

Each harness is an engine, one bounded call to it is one step, and the step’s result is what the workflow reads. Each call starts fresh, so nothing leaks from one step to the next except what the workflow hands over. The smallest use binds one engine to one job:

examples/one-agent-job.ts

import { agentJob, run } from '@obversa/runtime';
import { MockEngine } from '@obversa/runtime/testing';

const engine = new MockEngine(() => 'ready');

const job = agentJob({
  label: 'prepare-item',
  engine: 'offline',
  prompt: 'Prepare the item.',
});

const result = await run(job, {
  engine: 'offline',
  engines: { offline: engine },
});

console.log(result.outcome.status);

The engine here is a stand-in from the testing module, so the file runs offline. In a real team, engines.claude('claude-sonnet-4-5') or engines.codex('gpt-5.6-luna') takes its place, and Plugins and tools says how each one runs a request.

Inner and outer

Layer	What it is	Where it lives
The model	The weights behind an API.	The provider.
The agent harness	The loop, tools, memory and guardrails around one model.	Claude Code, Codex, OpenCode, Grok CLI.
The workflow	Several harnesses given roles, with reviews that run a step again and a person at the gate.	Obversa.

The word “harness” names both the inner and the outer layer. Harness engineering usually means building the inner loop. Obversa is the layer above it. In a workflow, give each review to a harness running another model family: workflow() refuses a reviewer from the same family as the writer it reviews.

Next steps

What is a meta-harness?: the outer layer.
Plugins and tools: the harnesses Obversa drives.
Installation: the file above, run for real.

Start

Concepts

Patterns

Examples

Evals and Gates

Recording Runs

Driving Runs

Surfaces and Hosts

Writing Workflow Shapes

Memory

Workspace

Compare

FAQ

API Reference

What Is an Agent Harness?

In Obversa

Inner and outer

Next steps

​In Obversa

​Inner and outer

​Next steps

In Obversa

Inner and outer

Next steps